Thank you for your reply, and for letting us know you still have the problem.
Sorry that it seems you just don't have the "Select all" option in iOS 12.
It could help to update your iOS software.
Update iOS 12 software on iPad
https://support.apple.com/en-au/guide/ipad/ipad9a74c576/ios
About iOS 15 update
https://support.apple.com/ios/update
OR
Would you be able to delete the large number e-mails more easily
by logging in to your eMail account on your eMail provider's website (Google. Yahoo etc) ?
Another option may be to just delete the Mail app, along with all the data,
.... you might need to have an old back-up (without so many eMails) of the iOS12 Mail app , to re-install.
How to delete built-in Apple apps on your iOS 12
https://support.apple.com/en-au/guide/ipad/ipad0bf80ff4/ios
Else:
Seems with iOS12, you may have to just, while viewing a list of emails:
Tap Edit, select as many emails as you can, then tap Trash.
(To turn the "confirmation of deletion" on or off, go to Settings > Mail > Ask Before Deleting.)
If you need to contact Apple:
There is a "Get Support" link at top right on this discussion page .... (eg: online chat & other options).